Multiple invertebrates and several vertebrate species are known to possess the ability to regenerate lost body parts (Goss, Clin. Orthop., 1980, 151:270–282; Kawamura and Fujiwara, Sem. Cell Biol., 1995,6:117–126; Tsonis, Devel. Biol., 2000,221:273–284). Thus, invertebrates can reconstruct the whole body from small pieces (Kawarnura and Fujiwara, supra). Examples of regeneration in vertebrates include (i) rabbits and bats which can fill in holes punched through their ears; (ii) adult salamanders which can regenerate a complete limb after amputation; and (iii) mice which can replace the tip of a foretoe when it is amputated distal to the last joint (Goss and Grimes, Am. Zool., 1972, 12:151; Neufeld and Zhao, pp. 243–252, In: Limb Development and Regeneration, Fallon ed., John Wiley and Sons, 1993).
In humans, the fingertips of young children have also been shown to regrow after amputation distal to the last joint (Goss, supra; Illingworth, J. Ped. Surg., 1974, 9:853–858). Two factors have been shown to be important for this regeneration: (1) the opened surface of a fresh wound that can be covered by epidermal epithelium originating from the margins of the amputation site (Stocum, pp. 32–53, In: Regulation of Vertebrate Limb Regeneration, Sicard ed., Oxford Univ. Press, 1985), and (2) an adequate nerve supply at the wound surface (Singer et al., Anat. Embryol., 1987, 177:29–36).
The cellular mechanisms underlying regeneration have been studied for a number of years, and there appear to be some conserved features between species. In vertebrates, there are two ways in which regeneration occurs. In some tissues, multipotent quiescent stem cells become activated by damage and proliferate to produce new cells of several different terminally differentiated phenotypes. Alternatively, there may be a change in the phenotype of the functional, fully differentiated cells, such that they lose many of their differentiated characteristics, and proliferate to form new fully differentiated cells of other phenotype. This latter process has been termed “transdifferentiation” (Okada, pp.349–380, In: Current Topics in Developmental Biology, Denis-Donini et al. eds., Acad. Press, 1980; Okada, Trans-differentiation, Oxford Sci. Publ., 1991).
Retinal regeneration represents an example of the regenerative process that can occur either through stem cells or via transdifferentiation, depending on the species. Thus, teleost fish contain a population of retinal progenitor stem cells that can act as a source of new retinal neurons following damage (Hitchcock and Raymond, Trends Neurosci., 1992, 15:103–108). In contrast, amphibians and embryonic chicks can regenerate their retina by a process that involves transdifferentiation of the cells in the pigment epithelium (RPE) to neural retinal progenitors (Reh and Pittack, Sem. Cell Biol., 1995, 6:137–142).
The existence of regeneration by transdifferentiation was questioned for a long time as it was not consistent with the classic view of differentiation, according to which a once acquired cellular phenotype was considered to be fixed due to irreversible changes in the gene expression pattern. However, the development of in vitro cell culture systems allowed the unequivocal experimental demonstration of regeneration by transdifferentiation. Thus, it has been shown that cultured fully differentiated pigmented epithelial cells of adult newt iris have the ability to dedifferentiate and proliferate to form a new tissue, lens (Eguchi et al., Proc. Natl. Acad. Sci. USA, 1974, 70:5052–5056; Abe and Eguchi, Dev. Growth Diff., 1977, 19:309–317).
Both in vivo and in vitro studies have demonstrated that cytoplasmic signals and changes in the gene expression (e.g., selective gene activation and/or silencing) caused by interactions with growth factors and components of the extracellular matrix are important in the control of cellular transdifferentiation (Kodama and Eguchi, Sem. Cell Biol., 1995, 6:143–149; Rao and Reddy, ibid., 151–156). Thus, it has been shown that copper deficiency in rats leads to loss of cell-cell interactions, altered microenvironment and global apoptosis of acinar cells in the pancreas which, in turn, causes oval and ductal pancreatic cells to undergo active proliferation resulting in their transdifferentiation into liver hepatocytes (Rao and Reddy, supra). In another series of experiments conducted with the neural crest-derived pigmented skin cells (chromatophores) of the Axolotl (Anibystoma mexicanum), it has been shown that the addition of guanosine can cause these cells to transdifferentiate from one pigmented cell type to another (Frost et al., Pigm. Cell Res., 1987, 1:3743; Thibaudeau and Holder, Pigm. Cell Res., 1998, 11:38–44).
It is believed that the replacement of complex appendages (i.e., epimorphic regeneration) following amputation in lower vertebrates also occurs by transdifferentiation (Goss, supra). Thus, during epimorphic regeneration, epidermal wound healing is followed by the accumulation of dedifferentiated blastemal cells beneath the wound epidermis. These blastemal cells are thought to originate by the dedifferentiation of the mesenchymal and Schwann cells of the stump tissue (Brockes, Science, 1984, 225:1280–1287) which then redifferentiate to reconstruct the limb tissue (Singer et al., Anat. Embyol., 1987, 177:29–36).